wikipedia讲http keepalive选项作用的图

HTTP_persistent_connection.svg‎

 

keepalive的时间范围内，这些http请求共用同一个TCP connection. 如果keepalive为off的话，则每次http请求都需要新建tcp connection

摘自wikipedia：
HTTP persistent connections, also called HTTP keep-alive, or HTTP connection reuse, is the idea of using the same TCP connection to send and receive multiple HTTP requests/responses, as opposed to opening a new connection for every single request/response pair. 

Advantages
less CPU and memory usage (because fewer connections are open simultaneously)
enables HTTP pipelining of requests and responses
reduced network congestion (fewer TCP connections)
reduced latency in subsequent requests (no handshaking)
errors can be reported without the penalty of closing the TCP connection